Fuel injectors, which are essentially fuel on/off valves controlled by an electric signal, are available in two broad families characterized by their electrical impedance—low impedance and high impedance. The impedance of a fuel injector dictates how much electric current will flow through it when it is connected across vehicle battery voltage (typically 12Vdc). Lower impedance results in a larger flow of electric current, and the larger electric current flow in turn provides more force to open the fuel injector. Thus, a low impedance fuel injector has more opening force than a high impedance fuel injector of an equivalent fuel injector flow rate.
Fuel injector flow rate is a measure of the quantity of fuel that can pass through a fully open fuel injector per unit of time, at a specified fuel pressure. The unit of measure commonly used in the United State for fuel injector flow rate is pounds of fuel per hour (lb/hr). The flow rate measurement is typically made at a fuel pressure of 43.5 pounds per square inch (psi). While fuel injector flow rate is a well-characterized parameter, it only applies to a fuel injector that is fully open. The fuel flow rates during the closed-to-open and open-to-closed transitions are generally not specified. In order to optimize engine performance (i.e., minimize emissions and fuel consumption, and maximize the power delivered per unit of fuel consumed), the total amount of fuel delivered during a fuel injector closed-open-closed cycle must be known. As discussed above, while information related to the fuel flow during transitions may not be available, the engine performance may be optimized if the time required for the transitions (i.e., closed-to-open, and open-to-closed) is minimized.
Low-impedance fuel injectors offer two important advantages over the high-impedance fuel injectors installed in most vehicles as original equipment. First, the higher electric current flowing through a low-impedance fuel injector enables it to open more quickly than a high impedance fuel injector of equivalent flow rating, resulting in a more precise control over fuel delivery, especially in situations where fuel demand is low, such as engine idling or driving at moderate speeds. Further, more precise fuel control enables a decrease in vehicle emissions and an increase in fuel efficiency.
Additionally, low-impedance injectors are available in a much wider range of fuel injector flow rates than the range available in high-impedance fuel injector technology. The relatively small electric current flowing through a high impedance injector limits the amount of force available to open it. This force limitation constrains the size of the fluid flow control mechanism inside the high impedance fuel injector which, in turn, constrains the maximum fuel flow rate. By contrast, low impedance fuel injectors offer roughly four times the amount of electric current compared to high impedance fuel injectors, enabling a significantly wider range of fuel flow rates. In fact, the largest readily available low impedance fuel injector has more than three times the flow rate of the largest high impedance fuel injector.
Despite the advantages of the low impedance fuel injectors, high impedance fuel injectors are more commonly used in commercially available vehicles. This is due to the much higher cost for the electronic circuitry used to operate the low impedance fuel injectors. Indeed, low impedance fuel injectors require both more sophisticated control, and higher electric current capacity, than high impedance fuel injectors, which, in turn, translates to higher cost.
As discussed above, a fuel injector is fluid flow control valve that is turned on by applying an electric current through its electric terminals, and turned off by removing the electric current. For many commercially available vehicles, this electric current is controlled by a computer, hereinafter referred to as the Engine Control Computer. The typical installation of fuel injectors on vehicles available, for example, in the United States, has one of the two fuel injector terminals connected to a source of battery voltage (nominally 12 Vdc), and the other fuel injector terminal connected to an Engine Control Computer output terminal.
To open a particular fuel injector, the Engine Control Computer temporarily connects its output terminal for that fuel injector to a battery ground terminal (nominally 0 Vdc). This temporary connection to the ground terminal typically is made inside the Engine Control Computer itself. The temporary connection to the ground terminal enables electric current to flow through the fuel injector, thus causing the fuel injector to open. To close the particular fuel injector, the Engine Control Computer removes the connection to the battery ground terminal for that fuel injector, which stops the flow of electric current through the fuel injector, resulting in the fuel injector closing.
The temporary connection to the battery ground terminal discussed above is generally referred to as a “pulse”. Furthermore, the total length of time for the temporary connection to the battery ground terminal is generally referred to as the “pulsewidth”. The Engine Control Computer controls the amount of fuel delivered to the engine by the fuel injector through the control of the duration of the pulsewidth. Typically, pulsewidths are in the range of 1.5 millisecond to 20 milliseconds. Also, the pulsewidth must account for the time needed for the fuel injector closed-to-open and open-to-closed transitions, even though the duration of those transitions may not be precisely predictable.
Vehicle manufacturers generally configure their Engine Control Computers to provide fuel injector pulsewidths that are appropriate for the particular engine under the expected range of operating conditions. However, due to manufacturing tolerance variability, the provided pulsewidths may not be suitable for every vehicle in all environmental operating conditions. For example, if the pulsewidths created by the Engine Control Computer are too short, the vehicle engine may not receive sufficient fuel for proper vehicle operation under unusually heavy loads, such as towing a trailer up a long incline, and may be seriously damaged as a result. On the other hand, if the pulsewidths are too long, the engine may receive too much fuel, which will likely result in a decrease in fuel economy and an increase in pollution. Given this, the ability to modify the pulsewidths generated by the Engine Control Computer would allow for optimization of the fuel delivery characteristics of one's vehicle.
High Impedance fuel injectors are very easy to control—this is their primary market advantage. To turn a high impedance fuel injector on, one needs only to connect one fuel injector terminal to a source of battery voltage (nominally 12 Vdc) and the other terminal to battery ground (nominally 0 Vdc). The high electrical impedance of the high impedance fuel injector inherently limits the electric current flowing through the fuel injector, and the circuit that is operating it, to approximately one ampere. This amount of electric current is small enough to prevent the fuel injector from overheating, even if it were to be turned on indefinitely. The one ampere operating current can be controlled by an inexpensive transistor in the Engine Control Computer. Further, to turn a high impedance fuel injector off, one simply opens the connection to one or both of the fuel injector terminals. In most cases, the fuel injector terminal connected to battery ground is the one that is switched on and off to control the fuel injector. The other fuel injector terminal is continuously connected directly to a source of battery voltage. It should be noted that the source of continuous battery voltage is typically controlled by the engine ignition such that battery voltage is applied to the fuel injector only when the engine ignition is on.
As discussed above, the control scheme for a high impedance fuel injector is simply an electrical switch between the one of the fuel injector's electric terminals and battery ground. The Engine Control Computer controls fuel flow through the fuel injector by closing the electric switch. When the Engine Control Computer opens the electric switch, fuel flow through the fuel injector ceases.
Low impedance fuel injectors require a more sophisticated control scheme. This is because their low electric impedance allows much more current to flow when the fuel injector is on. As was the case for the high impedance fuel injector, a low impedance fuel injector is turned on by connecting one of the fuel injector electric terminals to a source of battery voltage (nominally 12 Vdc) and the other terminal to battery ground (nominally 0 Vdc). This causes the electric current through the fuel injector to increase very rapidly, just as it does for the high impedance fuel injector. However, the electrical impedance of the low impedance fuel injector is too small to limit the electric current to a safe level. If the electric current was not controlled in some way, a low impedance fuel injector connected directly to battery voltage and ground would overheat and fail catastrophically in minutes.
Thus, a mechanism or approach to control the maximum current flowing though a low impedance fuel injector is desired. This maximum current, referred to as the “peak” current, is typically on the order of 4 amperes. It is this peak current, which greatly exceeds the current flowing through a high impedance fuel injector, that gives the low impedance fuel injector the added force it needs to open more quickly than a high impedance fuel injector of an equivalent flow rate, and/or to open larger fluid flow control valves than a high impedance fuel injector can operate. However, the peak current may cause a low impedance fuel injector to overheat and fail if it persists for too long. Thus, a further control mechanism or approach is desired to decrease the electric current from the peak value used to open the fuel injector to the smaller amount of current, referred to as the “hold” current, needed to hold it open. This hold current is typically on the order on 1 ampere, the same as the current flowing through a high impedance fuel injector. The peak current is typically allowed to persist for approximately 1 millisecond. The hold current then persists until the Engine Control Computer disconnects the fuel injector from battery ground, causing the fuel injector to close.
In other words, the low impedance fuel injector must be operated using a “peak” and “hold” electric current control scheme. In order to control the amount of electric current flowing through the fuel injector, the current must be measured and the measurement result used to operate a variable electric restriction. This is much more complicated, and thus more expensive, than the simple on/off control scheme required by high impedance fuel injectors. In addition, electric components exposed to the 4 amperes (or possibly more) of electric current must be significantly more robust than components that are only exposed to 1 ampere. This adds more cost to the peak and hold fuel injector control system.
FIGS. 1A–1B are block diagrams illustrating a standard connection of an Engine Control Computer and fuel injectors, and a standard batch-fire connection of the Engine Control Computer and fuel injectors, respectively. Referring now to FIG. 1A, there is shown an Engine Control Computer 101 operatively coupled to a plurality of fuel injectors 102 of a vehicle engine by corresponding respective fuel injector control wires 103. The configuration shown in FIG. 1A typically is provided with the vehicles manufactured after early 1990s. In most mass-marketed automobiles, there is a single fuel injector for each cylinder in the engine. Thus, a 4-cylinder engine typically has four fuel injectors, a 6-cylinder engine typically has six fuel injectors, and so on. Referring again to FIG. 1A, a 4 cylinder Engine Control Computer 101 would correspondingly have four output terminals each coupled to a corresponding one of the fuel injector control wire 103, each separately connected to a respective fuel injectors 102.
Most modern vehicles use a single Engine Control Computer output terminal to control a single fuel injector as shown in FIG. 1A. However, some older vehicles use a simpler scheme in which a single Engine Control Computer output operates two or more fuel injectors simultaneously. This approach, sometimes referred to as “batch fire”, as shown in FIG. 1B. Referring now to FIG. 1B, as shown, each fuel injector control wire 104 may be connected to one or more respective fuel injectors 102. For example, as shown in FIG. 1B, each of the fuel injector control wires 104 are connected to the same number of fuel injectors 102.
One advantage of the batch fire configuration is that it includes comparatively includes lower cost electronics. The older, inexpensive Engine Control Computers did not operate fast enough to control one fuel injector per cylinder. Even though batch fire systems do not operate the fuel injector for each cylinder at precisely the right time, their performance was sufficient to meet the emission standards of the time. Referring back to the Figures, the configuration shown in FIG. 1A is typically “sequential” in that the fuel injectors are operated in sequence, at the precise moment in time that the particular cylinder is ready to accept fuel and air. By contrast, the batch fire configuration shown in FIG. 1B may operate one fuel injector in the batch at the right time, while the remaining fuel injectors in the same batch are operated “out of sequence” with respect to their combustion cycle (intake-compression-ignition-exhaust).
The automotive aftermarket offers Engine Control Computers capable of operating low-impedance fuel injectors, but their costs are relatively high, for example, ranging from more than $1,000 to several thousands of dollars. Moreover, while commercial software in the automotive aftermarket is available which would allow the vehicle owner to optimize the fuel injector pulsewidths for his or her particular vehicle, such commercial software is not compatible to the use of low impedance fuel injectors with the original equipment Engine Control Computer.
In view of the foregoing, it would be desirable to have a system and method for retrofitting a low impedance fuel injection system to an internal combustion engine for which the original system was designed with a high impedance fuel injection system.